Black rubber wheels for ROTF Leader Megatron

Does anybody know where I can find black rubber rings/washers/wheels that are the same size as the gray plastic wheels (circled in red) on ROTF Leader class Megatron?

The reason I ask is that when he is in his alternate mode, he won't slide across smooth surfaces. Plus, black rubber rings/washers/wheels will blend in with his black tank treads.

The reason I ask is that when he is in his alternate mode, he won't slide across smooth surfaces. Plus, black rubber rings/washers/wheels will blend in with his black tank treads.